"Self-induction stops the voltage rise in inductive circuits." If your work or hobby is connected with electricity, you have probably heard such statements. In fact, this phenomenon is inherent in inductive circuits, both explicitly, for example, coils, and implicitly, such as parasitic parameters of the cable. In this article, we will explain in simple terms what self-induction is and where it is applied.
Content:
- Definition
- Inductance
- Transformer and mutual induction
- Benefit and harm
- Conclusion
Definition
Self-induction is the appearance in a conductor of an electromotive force (EMF) directed in the opposite direction relative to the voltage of the power source when current flows. Moreover, it occurs at the moment when the current in the circuit changes. A changing electric current generates a changing magnetic field, which in turn induces an EMF in the conductor.
This is similar to the formulation of Faraday's law of electromagnetic induction, which says:
When a magnetic flux passes through a conductor, an EMF arises in the latter. It is proportional to the rate of change of the magnetic flux (mat. time derivative).
That is:
E = dФ / dt,
Where E is the EMF of self-induction, measured in volts, F is the magnetic flux, the unit of measurement is Wb (weber, it is also equal to V / s)
Inductance
We have already said that self-induction is inherent in inductive circuits, therefore, we will consider the phenomenon of self-induction using the example of an inductor.
An inductor is an element that is an insulated conductor coil. To increase the inductance, the number of turns is increased or a core made of soft magnetic or other material is placed inside the coil.
The unit of measure for inductance is Henry (H). Inductance refers to how strongly a conductor resists an electric current. Since around each conductor through which a current flows, a magnetic field is formed, and if you place a conductor in an alternating field, a current will arise in it. In turn, the magnetic fields of each turn of the coil are added. Then a strong magnetic field will arise around the coil through which the current flows. When its strength changes in the coil, the magnetic flux around it will also change.
According to Faraday's law of electromagnetic induction, if an alternating magnetic flux penetrates the coil, then a current and EMF of self-induction will arise in it. They will prevent the current from flowing in the inductor from the power supply to the load. They are also called self-induction EMF extracurrents.
The EMF formula of self-inductance on inductance is:
That is, the greater the inductance, and the more and faster the current has changed, the stronger the EMF surge will be.
With an increase in the current in the coil, an EMF of self-induction arises, which is directed against the voltage of the power source, respectively, the increase in current will slow down. The same happens when decreasing - self-induction will lead to the appearance of an EMF, which will maintain the current in the coil in the same direction as before. It follows that the voltage at the terminals of the coil will be opposite to the polarity of the power supply.
In the figure below you can see that when the inductive circuit is turned on / off, the current does not arise abruptly, but changes gradually. The laws of commutation also speak about this.
Another definition of inductance sounds like this: the magnetic flux is proportional to the current, but in its formula, inductance acts as a coefficient of proportionality.
Ф = L * I
Transformer and mutual induction
If you place two coils in close proximity, for example, on one core, then the phenomenon of mutual induction will be observed. Let's pass an alternating current through the first, then its alternating current will penetrate the turns of the second and an EMF will appear on its outputs.
This EMF will depend on the length of the wire, respectively, the number of turns, as well as on the magnitude of the magnetic permeability of the medium. If you place them just next to each other, the EMF will be low, and if you take a core made of soft magnetic steel, the EMF will be much higher. Actually, this is how the transformer works.
Interesting: this mutual influence of the coils on each other is called inductive coupling.
Benefit and harm
If you understand the theoretical part, it is worth considering where the phenomenon of self-induction is applied in practice. Let's look at examples of what we see in everyday life and technology. One of the most useful applications is a transformer, we have already discussed the principle of its operation. Now they are found less and less, but previously, fluorescent tubular lamps were used daily in lamps. The principle of their work is based on the phenomenon of self-induction. You can see her schemes below.
After the voltage is applied, the current flows through the circuit: phase - choke - spiral - starter - spiral - zero.
Or vice versa (phase and zero). After the starter is triggered, its contacts open, then throttle (a coil with a large inductance) seeks to maintain the current in the same direction, induces a self-induction EMF of a large value and the lamps are ignited.
Similarly, this phenomenon applies to the ignition circuit of a car or motorcycle that runs on gasoline. In them, a mechanical (breaker) or semiconductor switch (transistor in the ECU) is installed in the gap between the inductor and the minus (mass). This key, at the moment when a spark must form in the cylinder to ignite the fuel, breaks the coil power supply circuit. Then the energy stored in the core of the coil causes an increase in the EMF of self-induction and the voltage across the electrode of the candle increases until a breakdown of the spark gap occurs, or until it burns out coil.
In power supplies and audio equipment, it is often necessary to remove excess ripple, noise or frequency from the signal. For this, filters of different configurations are used. One of the options is LC, LR filters. By preventing the growth of the current and the resistance of the alternating current, respectively, it is possible to achieve the set goals.
Self-induction EMF harm to contacts of switches, circuit breakers, sockets, circuit breakers and other things. You may have noticed that when you unplug the plug of a working vacuum cleaner from the outlet, a flash inside it is very often noticeable. This is the resistance to the change in current in the coil (motor winding in this case).
In semiconductor switches, the situation is more critical - even a small inductance in the circuit can lead to their breakdown, when the peak values of Uke or Usi are reached. To protect them, snubber circuits are installed, on which the energy of inductive bursts is dissipated.
Conclusion
Let's summarize. The conditions for the occurrence of self-induction EMF are: the presence of inductance in the circuit and a change in the current in the load. This can occur both in operation, when changing modes or disturbing influences, and when switching devices. This phenomenon can harm the contacts of relays and starters, as it leads to arcing when opening inductive circuits such as electric motors. To reduce the negative impact, most of the switching equipment is equipped with arc chutes.
For useful purposes, the EMF phenomenon is used quite often, from a filter to smooth out ripple current and frequency filter in audio equipment, to transformers and high-voltage ignition coils in cars.
Finally, we recommend watching a useful video on the topic, which briefly and in detail discusses the phenomenon of self-induction:
We hope you now understand what self-induction is, how it manifests itself and where it can be used. If you have any questions, ask them in the comments below the article!
Related materials:
- Properties and characteristics of the electric field
- Faraday's laws in chemistry and physics
- Distribution of charges in a conductor
